According to the BBC, at least eight people have been confirmed dead in the blast, which happened in Sassine Square, a busy part of the mostly Christian Ashrafiya district in the eastern part of the Lebanese capital.

The source said he hadn't yet confirmed whether Hassan was hit, but according to Reuters, at least one Lebanese TV station is reporting that Hassan was among those killed.

He is described as the head of Lebanon's internal security service. The Guardian said he recently led an investigation into a Lebanese former minister, Michel Samara, who supports Syria President Bashar al-Assad and has been accused of planning terrorist attacks.

The 19-month conflict in neighboring Syria has fueled tensions in Lebanon between supporters and opponents of Assad.

If Hassan's death is confirmed, the Guardian's Middle East correspondent, Martin Chulov, said it would be "the most dangerous event in Lebanon since assassination of [former Lebanese prime minister] Rafiq Hariri in 2005."

Hassan was closely involved in investigating Hariri's death and blamed the murder on Shiite militant group Hezbollah, according to the Washington Post. He remained a close ally of Saad Hariri, Sunni opposition leader, former prime minister and Rafiq's son.

"Wissam al-Hassan was a very important man in Lebanese politics, he was an important man in the great divide splitting the country and he was an important man in the police work that has uncovered many sensitive things," an adviser to Hariri, Mohammed Chatah, told the Post.

GlobalPost correspondent Hugh Macleod said from Beirut that Wissam Hassan is "the ISF official who led the sting operation against one of Bashar Assad's top men in Lebanon, Michel Samaha."

MacLeod continued in an e-mail: "Samaha was considered untouchable until he was arrested in August caught on video bringing bombs into Lebanon. The operation was led by the ISF, which is one of Lebanon's security branches allied with the pro-West anti-Syrian March 14 camp, as opposed to the March 8, Hezbollah-led, pro-Syrian alliance. If Hassan has been assassinated he will join the list of Lebanese officials who dared to investigate Syria's murky role in Lebanon, including Wissam Eid, who found Hezbollah phone records linking their members to killing of Rafik Hariri."

"There is a clear pattern in which anybody who opposes Damascus' malign role in Lebanon, whether through its heavy handed security occupation or its overt meddling in Lebanese politics, is assassinated," he wrote.

The BBC reported that opposition leader Saad Hariri accused Assad of being behind the bombing, a sentiment echoed by Druze leader Walid Jumblatt.

Lebanon's opposition March 14 bloc held Prime Minister Najib Mikati responsible for Hassan's death and called on him to resign, according to Reuters.

United Nations Secretary General Ban Ki-moon condemned the bombing as a "terrorist act," on Friday. He said in a statement, "At a time of regional turbulence, it is of the utmost importance that all parties ensure calm and restraint in order to preserve peace and stability in Lebanon."

The White House also condemned the bombing, labeling it a "terrorist attack." US National Security Council spokesman Tommy Vietor said, "The United States condemns in the strongest terms the terrorist attack today in Beirut that killed Lebanese Internal Security Forces Information Bureau Director Wissam al-Hassan and at least seven others, and wounded dozens more."

"There is no justification for using assassination as a political tool. The United States will stand by the Government of Lebanon and the people of Lebanon as they work to bring those responsible for this barbaric attack to justice and build a future where all Lebanese can live in security and dignity," he continued, according to Reuters.

Today’s car bomb blast occurred during rush hour when many parents were picking up their children from school, Reuters said. It was the first "major car bomb attack" in Beirut in four years, according to the BBC.

The Associated Press said one of its reporters at the scene saw “bloodied people being helped into ambulances and heavy damage” to nearby buildings.

Watch live video about the bombing here:

This video posted on YouTube purportedly shows the aftermath of the explosion.